1. Field of the Invention
This invention relates generally to power supplies, and more specifically, to a switched mode power supply.
2. Background Information
In many electronic device applications, especially the low power off-line adapter/charger market, an approximately constant voltage/constant current output characteristic is required. Known switched mode power supply circuits providing constant output current and voltage characteristics typically use circuitry on the secondary (or output side) of the power supply that sense output voltage and current and generate a feedback signal. The feedback signal is typically communicated to a regulator circuit on the primary side of the power supply through an opto coupler component. This feedback signal is then used to control the switching of a primary switch to provide the required power supply output characteristic.
The specific function of maintaining constant output current is normally achieved with secondary current sense resistors in series with the output load that provide a voltage signal proportional to the current flowing through the secondary current sense resistors. This voltage signal is then used to provide a feedback signal, which in turn is used to control the power switch.
Another technique to provide an approximately constant output current is to derive information about the output voltage of the power supply from an auxiliary winding of the power supply transformer on the primary side of the power supply. This information is then used to adjust the primary switch current limit threshold independent of any supply current required by the primary regulator circuit.
A regulator circuit maintaining an approximate constant current output characteristic is disclosed. In one aspect, a disclosed regulator circuit controls a switch that has a current limit threshold. A supply terminal and feedback terminal of the regulator are connected together as a control terminal such that a shunt regulator current is the control terminal current in excess of the internal consumption of the internal supply current of the regulator. The current limit threshold of the switch is changed as a function of the shunt regulator current. In one embodiment, the connection of the supply terminal and feedback terminal is external to the regulator. In another embodiment, the connection of the supply terminal and feedback terminal is internal to the regulator. In one embodiment, the current limit threshold of the switch is increased with increasing shunt regulator current. In another embodiment, the current limit threshold of the switch is decreased with increasing shunt regulator current. In one embodiment, the switch and regulator are integrated on a monolithic chip. In one embodiment, the switch is a metal oxide field effect transistor (MOSFET). In another embodiment, the switch is a bipolar transistor. In one embodiment, the regulator is used in a switching power supply. In one embodiment, the regulator is used in a switching power supply with an approximately constant output voltage and output current characteristic.
In another aspect, a disclosed regulator circuit controls a switch that has a current limit threshold. A control input of the regulator circuit receives a current that is the sum of the internal supply current consumed by the regulator circuit and a feedback current. The current limit threshold of the switch is changed as a function of the feedback current. In one embodiment, the current limit threshold of the switch is increased with increasing feedback current. In another embodiment, the current limit threshold of the switch is decreased with increasing feedback current. In one embodiment, the switch and regulator are integrated on to a monolithic chip. In one embodiment, the switch is a MOSFET. In another embodiment, the switch is a bipolar transistor. In one embodiment, the regulator is used in a switching power supply. In another embodiment, the regulator is used in a switching power supply with an approximately constant output voltage and output current characteristic. Additional features and benefits of the present invention will become apparent from the detailed description and figures set forth below.